Intelligent Subtractive Manufacturing

Not everyone involved in rapid manufacturing thinks additive fabrication systems will win over the hearts and minds of design engineers and manufacturers. Protomold President Brad Cleveland is placing his bets on what he calls "intelligent subtractive" manufacturing — or the use of interrelated design, CAM and operations software to streamline the many manufacturing steps that surround high-speed machining.

"We're banking on the combination of advancements in software algorithms, processing systems and off-the-shelf CNC machining technology over anything additive," says Cleveland, who earlier in his career actually co-founded a company that made additive metal components. 

His reasoning partly involves speed. "We just believe that for the foreseeable future, it will be much faster to cut materials than to build up parts layer by layer," he says. And he isn't just boasting when he talks about speed. Protomold's high-speed machining operations have gotten so fast that the company can supply injection-molded parts in a single business day, which means it takes Protomold only a fraction of a day to machine aluminum mold cores and cavities. Protomold's sister company, First Cut Prototype, has similarly short lead times for machined plastic parts.

Another reason why Cleveland believes additive processes won't become much more prominent than they are today boils down to materials. "There are severe limitations in materials available for additive processes," he says. "And many of the materials are sole source. Tell that to your purchasing department." Moreover, he points to the deficiencies in the surface finish and strength properties of additive parts as factors that will weight against them in applications that truly require molded or machined properties.

For all his seeming pessimism about additive processes, Cleveland believes they will have a role in the future — just not a particularly large one. He acknowledges additive processes can excel at mass customization and produce part geometries that subtractive methods cannot. But these two advantages translate only to niche applications in his view.  "For every part that is best made with an additive process I can show you 100 that are better made via intelligent subtractive processes," he says.

Strength Smackdown: Additive Fabrication Versus Molding and Machining

Protomold president Brad Cleveland believes that the mechanical properties of plastic parts made on additive machines don't come close enough to those of injection-molded and machined plastic parts, which creates design engineering difficulties. So Design News asked Cleveland to back up his assertions about material properties — and he did. Protomold hired an independent materials testing lab, Stork Materials Technology, to run ASTM tensile and flexural tests of three kinds of ABS parts — injection molded, CNC machined and fused deposition modeled (FDM) — on a Stratasys machine. The study tested the FDM part in different orientations to account for the anisotropy found in additive parts. It also tests the molded part in two different gating configurations. The study, whose results are summarized in these tables below and which can be downloaded in full here, shows a significant strength advantage for machined and molded ABS grades.